Integrated power systems are being developed that comprise fuel processors and fuel cells, that are to be operated in an integrated way to produce electric power from hydrogen-containing fuels. Fuel cells are well known, and are electrochemical devices that react hydrogen with oxygen to produce electric power. The hydrogen for the fuel cell can be produced by a fuel processor.
A fuel processor is a device that produces a product gas from hydrogen-containing fuel sources and water. Air may also be used to facilitate the process. The product gas contains hydrogen, and may include diluents such as carbon monoxide, carbon dioxide, nitrogen, oxygen, water, and other species. For fuel cell power systems, the most important requirement of the fuel processor is that it must generate a product gas (generally called “reformate”) of sufficient quality for reliable and efficient operation of the fuel cell. (Note that the reformate as defined herein includes the case of pure hydrogen.)
Until recently, fuel processors and fuel cells have been developed independently of each other, typically in different organizations. Each of these has been optimized in isolation. Fuel processors have been (and are being) optimized to produce reformate that is adapted for use by a particular type of fuel cell. The PEM (polymer electrolyte membrane) type of fuel cell is attractive for mobile or small applications, but it is particularly demanding in terms of reformate quality. The catalyst in a PEM fuel cell is readily poisoned by various materials potentially present in reformate, particularly CO (carbon monoxide), which is a common byproduct of the reforming process. Other types of fuel cells, including phosphoric acid fuel cells, also need reformate of a defined purity to avoid cell poisoning or other damage. Thus, development of fuel processors has focused on obtaining high quality reformate in an efficient way.
On the other hand, fuel cells have been developed with an entirely different set of objectives. Input of a suitable reformate is typically assumed. Much development effort is focused on optimization of the detailed design of the electrodes, heat exchangers, humidification equipment where required (for example, in most PEM cell designs), and overall system weight, power density, and reliability. Given the design of a fuel cell, characteristic parameters of the cell are determined. These include the polarization curves (voltage developed as a function of current drawn from the cell), the dependence of the polarization curves on hydrogen concentration in the cell (and any gradients in the concentration), and other known cell parameters.
It is not normally feasible for a fuel cell to completely consume the fuel (hydrogen), which is fed to it. In a typical integrated fuel cell/fuel reformer system (an “integrated system”), the exhaust from the anode side of the fuel cell, which contains the hydrogen that is not used, is fed back into the fuel processor to be burned as a source of energy. This energy is typically used to assist in the process of conversion of fuel (such as gasoline, jet fuel, diesel fuel, kerosene, methane, propane, ethanol or methanol) into reformate, as this process is generally endothermic (heat-absorbing).
In the initial stage of system integration, the degree of hydrogen usage in the fuel cell is typically chosen based on fuel processor energy requirements and practical heat exchanger sizes for the particular intended use, and then the system is operated so as to maintain high efficiency in each of the fuel cell and fuel reformer. In conducting such integration, it has become clear that this approach neglects impacts of stoichiometry and polarization on the system efficiency, and generally neglects optimization of the system given a level of anode gas return. Hence, it is not clear what the optimum ratio of anode bypass to input hydrogen, or the preferred selection of the voltage and current for a particular power output, should be for efficient operation of the system. Moreover, since there are many interrelated variables, it is not straightforward to determine how to operate the system with maximal efficiency.
There are numerous descriptions of electronic control of subcomponents of an integrated system, and even general descriptions of control methods for the system as a whole. However, none of these describes how to calculate the set of system parameters needed for operating the entire system in the most efficient mode.